Cruise control is now usual in motor vehicles, e.g. cars, trucks and buses. An object of cruise control is to achieve a uniform predetermined speed. This is done either by adjusting the engine torque to avoid retardation, or by applying braking action on downhill runs where the vehicle is accelerated by its own weight. A more general object of cruise control is to provide convenient driving and better comfort for the vehicle's driver. A driver of a vehicle equipped with cruise control usually chooses a set speed vset as the speed he/she wishes the vehicle to maintain on level roads. A cruise control then supplies an engine system of the vehicle with a reference speed vref used for control of the engine. The set speed vset may thus be regarded as an input signal to the cruise control, whereas the reference speed vref may be regarded as an output signal from the cruise control and is used for control of the engine.
Today's traditional cruise control (CC) maintains a constant reference speed vref usually set by the vehicle's driver in the form of a set speed vset which is thus here a desired speed chosen for example by him/her, and for today's conventional cruise controls the reference speed is constant and equal to the set speed, i.e. vref=vset. The value of the reference speed vref changes only when adjusted by the driver while the vehicle is in motion. The reference speed vref is then sent to a control system which controls the vehicle so that its speed corresponds when possible to the reference speed vref. If the vehicle is equipped with an automatic gearchange system, the gears may be changed by that system on the basis of the reference speed vref to enable the vehicle to maintain the reference speed vref, i.e. to enable it to maintain the desired set speed vset.
In hilly terrain, the cruise control system will try to maintain the set speed vset uphill and downhill. This may result inter alia in the vehicle accelerating over the crest of a hill and into a subsequent downgrade. It will then need to be braked to avoid exceeding the set speed vset or will reach a speed vkfb at which the constant speed brake is activated, which is a fuel-expensive way of driving the vehicle. It may also need to be braked downhill to avoid exceeding the set speed vset or the constant speed brake's activation speed vkfb in cases where the vehicle does not accelerate over the crest of the hill.
To reduce fuel consumption, especially on hilly roads, economical cruise controls such as Scania's Ecocruise® have been developed. This cruise control tries to estimate the vehicle's current running resistance and also has information about its historical running resistance. The economical cruise control may also be provided with map data comprising topographical information. The vehicle is then located on the map, e.g. by means of GPS, and the running resistance along the road ahead is estimated. The vehicle's reference speed vref can thus be optimised for different types of roads in order to save fuel, in which case the reference speed vref may differ from the set speed vset. This specification refers to cruise controls which allow the reference speed vref to differ from the set speed vset chosen by the driver, i.e. reference speed-regulating cruise controls.
An example of a further development of an economical cruise control is a “look ahead” cruise control (LACC), a strategic form of cruise control which uses knowledge of sections of road ahead, i.e. knowledge of the nature of the road ahead, to determine the reference speed vref. LACC is thus an example of a reference speed-regulating cruise control whereby the reference speed vref is allowed, within a certain range (vmin, vmax), to differ from the set speed vset chosen by the driver, in order to achieve more fuel saving.
Knowledge of the road section ahead may for example comprise information about prevailing topology, road curvature, traffic situation, roadworks, traffic density and state of road. It may further comprise a speed limit on the section ahead, and a traffic sign beside the road. Such knowledge is for example available from location information, e.g. GPS (global positioning system) information, map information and/or topographical map information, weather reports, information communicated between vehicles and information provided by radio. All this knowledge may be used in a variety of ways. For example, information about a speed limit on the road ahead may be used to achieve fuel efficiency by lowering the vehicle's speed before reaching a lower speed limit. Similarly, knowledge of a road sign which indicates for example a roundabout or intersection ahead may also be used to achieve fuel efficiency by braking before the vehicle reaches the roundabout or intersection.
An LACC cruise control does for example make it possible, before a steep upgrade, for the reference speed vref to be raised to a level above the set speed vset, since the vehicle will be expected to lose speed on such a climb owing to high train weight relative to engine performance. Similarly, before a steep downgrade, the LACC cruise control makes it possible for the reference speed vref to be lowered to a level below the set speed vset, since the vehicle will be expected (predicted) to accelerate on such a downgrade owing to its high train weight. The concept here is that reducing the speed at which the vehicle begins the downhill run makes it possible to reduce the energy braked away and/or the air resistance losses (as reflected in the amount of fuel injected before the downgrade). The LACC cruise control may thus reduce fuel consumption without substantially affecting journey time.
Such cruise control systems can be made robust if the topography ahead is made known by the vehicle having map data and GPS. The cruise control system may also alter the vehicle's speed proactively, i.e. before situations arise.
A vehicle usually has a number of ECUs (electronic control units) which control various electronic systems of the vehicle. The vehicle's engine is often controlled by an ECU of its own, called EMS (engine management system). A cruise control's logic may be situated in the EMS but this is sometimes not possible where the EMS has insufficient memory capacity and/or already has a high processor load. If the cruise control's logic is situated in some other ECU than the EMS, reference values, e.g. a desired reference speed vref, have to be sent via CAN (controller area network) to the engine control system's regulator, which then regulates the vehicle's speed on the reference speed vref.
A traditional PID regulator works on the basis of a reference speed vref received. When this reference speed vref is altered by the cruise control's logic and is sent via CAN, it is the PID regulator in the engine control system which regulates the vehicle speed towards the reference speed vref. The cruise control logic predicts the vehicle's speed but the regulator in the engine control system tries at the same time to regulate the vehicle's speed, which can lead to problems. It may for example result in maximum torque not being ordered by the engine system at the beginning of a climb despite the cruise control logic having catered for it in the prediction of the reference speed vref. There is therefore risk that the regulator may regulate the engine system with a gradually increasing error.
US 2005/0096183 refers to a speed regulator for a vehicle travelling downhill. The hills concerned are described as having a particular downward gradient, and when the driver operates a gradient switch a constant speed is set for the vehicle for as long as the switch is on. A constant speed is thus set when the driver indicates that the vehicle is on a hill.
U.S. Pat. No. 6,076,036 bases cruise control on a speed setting, the vehicle's current speed, an acceleration and a change in the gradient of the road, which is measured by a sensor, in order to set the fuel flow for lower fuel consumption.